Conventionally, there has been proposed a seat for a vehicle provided with a mechanism for moving a head rest to a front side of the vehicle, for protecting a head portion, a neck portion or the like of an occupant at a time of a collision from a rear side of the vehicle (for example, refer to Japanese Unexamined Patent Publication Nos. 2000-211410 and 2003-54343).
In the case that an impact is applied to the vehicle from the rear side, an upper half body of the occupant is constrained to a seat back, however, the neck portion which is not constrained is moved to the rear side in retard of the upper half body. At this time, there is a risk that a load is applied to the neck portion, however, if the head rest is moved to a front side of the vehicle with respect to the seat back in this case, the head portion of the occupant is held down. As a result the load applied to the neck portion of the occupant is reduced.